Summary: In automatic compaction, if a compaction is bottommost, it goes to bottom thread pool. We should do the same for manual compaction too. Pull Request resolved: https://github.com/facebook/rocksdb/pull/6593 Test Plan: Add a unit test. See all existing tests pass. Reviewed By: ajkr Differential Revision: D20637408 fbshipit-source-id: cb03031e8f895085f7acf6d2d65e69e84c9ddef3